Wide angle biocular eyepiece

ABSTRACT

A wide angle biocular eyepiece comprising four or five spaced optical elements.

r TUQ United Sta 51 3,658,412 Seaman 1 Apr. 25, 1972 [54] WIDE ANGLEBIOCULAR EYEPIECE [56] References Cited [72] inventor: William J.Senmnn, Center Line, Mich. UMTED STATES PATENTS [73] Assign e: hry l r pn. gh P k. 2,549.158 4/1951 Benele ..350 1 75 E Mich. 3,516,735 6/l970Goodell ..350/220 [22] Filed: Primary Examiner-John K. Corbin [2|] APPL7 2 3 Anorney--Talburtt and Baldwin [57] ABSTRACT 250/2 1 A wide anglebiocuiar eyepiece comprising four or five spaced l l 5s 1 Field arse-mi"350/175 E, 220, 219, 223 6 6 Claims, 1 Drawing Figure BACKGROUNDEyepieces used in visual systems instruments usually have an externalaccessible pupil location where the eye may be placed for the purpose ofseeing a large apparent field. Only the rays passing through this welldefined exit pupil need be optically corrected. With such a smallaperture, the system need only be corrected for astigmatism, fieldcurvature, distortion and lateral color. Spherical aberration, coma andaxial chromatic aberration are normally present but the instrumentselects so small a portion of the possible rays passing through theeyepiece that these aberrations ordinarily are not significant andcorrection need not be made for them.

Night vision devices are one type of optical instrument on whicheyepieces are used. In both active and passive night vision devices, avisible image is formed on a phosphor luminescent screen with detailstoo small for convenient viewing by the unaided eye. Optical means arerequired to present an enlarged image to an observer.

Many types of optical systems can be used to observe images formed inelectro-optical devices where the photon image appears on a phosphorscreen. Most common are the monocular eyepieces, which function asmagnifiers and are usually built by enlarging the useful area of theoptical elements. The resulting optical systems suffer from intolerableaberrations caused by the lens curvatures being penetrated by light raysat great distances from the optical axis. This ray distance is at leastas large as one-half of the interpupillary distance of the observer.Considering the relatively short focal length required to producesufficient magnification in such optical systems, the relative apertureat which the biocular magnifier must operate is often smaller than l .0.Correction of optical aberrations for this large relative aperture isdifficult at best, and becomes impossible if the number of lens elementsavailable for aberration correction is limited by weight or sizeconstraints. Many well corrected biocular magnifiers have been describedin the literature. Two typical examples are shown in U.S. Pat. Nos.2,885,928 and 2,900,871. From the description of the optical systemgiven in these patents it is apparent that biocular eyepieces require aconsiderable design effort to provide optical aberration correctionacross the necessarily large exit pupil diameter.

When eyepieces are used with an image tube, the object plane radiateslight in all directions and there is no well defined pupil or stop. If atypical prior art eyepiece is used and is viewed with the eye displacedeither longitudinally or laterally, the observer will see considerableamounts of distortion, astigmatism, and color aberration.

This will also be the case when both eyes are used in viewing. Here eacheye is displaced laterally from the optical axis and may be lookingthrough different portions at either side of the eyepiece so that theaberrations will not only produce distortions but will blur the imageand disturb steriopsis as well.

As a rule, eyepieces which are sufficiently corrected for a small, fixedpupil usually prove to be inadequate when used as magnifiers. What isrequired is a system that is uniformly good over a large pupil.

Night observation and other viewing through eyepieces can beaccomplished much more easily if the observer is permitted to use botheyes for the viewing function without these aberrations. In addition,for prolonged viewing without eye strain, it is necessary that the userobserve the image with both eyes.

It is also desirable from a human factors viewpoint to supply as large afield as possible, with a large exit pupil and eye relief. Normally,however, large exit pupils and large fields of view introduce severegeometric aberrations, among them, astigmatism, lateral color anddistortion. Spherical aberration becomes a factor on axis if the exitpupil is chosen too large. All these aberrations tend to decrease thetheoretical and practical contrast transfer of the eyepiece assembly andmust be corrected.

SUMMARY 01- THE lNVENTlON The present invention relates in general tofour or five element eyepieces of sufficient aperture to allow themagnified observation of intermediate images with both eyes of anobserver simultaneously to obtain a sharp image across large fields ofview. The eyepieces allow for head and eye movement. The bioculareyepieces described herein relate specifically to the observation ofimages formed on phosphor screens of electronic imaging devices, but maybe used for other purposes where biocular viewing is desired. Inaddition to providing sharp imagery, the eyepieces described herein areuseful for large fields of view.

More specifically, this invention provides eyepieces with an external,accessible pupil of such large diameter that the eyes of the observermay comfortably view the image even if not perfectly centered with theoptical axis. In such eyepieces all of the emergent rays are adequatelycorrected, so that it is unimportant between relatively wide limitswhere the observer's eyes are located either laterally orlongitudinally.

It is then a primary object of this invention to provide a wide anglebiocular eyepiece suitable for use with image intensifier tubes in nightvision equipment to obviate the eye strain which is often encounteredwith monocular active or passive night observation devices.

It is a general object to provide biocular eyepieces for viewingintermediate images through a single optical channel, that is, throughone set of coaxial optical elements.

It is a specific object of this invention to provide biocular eyepiecessuitable for magnified viewing of phosphor screens of electro-opticalimaging devices, such screens having diameters from 25 to millimetersand emitting visible light of the spectral characteristics representedby the P-20 phosphor curve. The peak of the P-20 spectral distributionis near 5,500A, closely represented by the green e-line of Mercury.

It is a further object to provide eyepieces with the dispersive qualityof the optical elements selected so as to provide a particularly sharpachromatic image when used with light sources with a spectraldistribution similar to that of a P-20 type phosphor.

It is a still further object of this invention to provide widefield,biocular eyepieces with an apparent field of view of approximately 50and a long eye relief of approximately 75 millimeters for use with imageintensifier tubes having a P-ZO phosphor.

It is yet another object of this invention to provide spherically andchromatically corrected biocular eyepieces with 80 mm exit pupildiameter and having an eye relief of not less than 80 mm.

BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention aswell as other objects and advantages thereof will be apparent fromconsideration of the following specification and the single FIGURE ofthe drawing which illustrates a diagramatic embodiment of an eyepieceaccording to this invention in combination with an image tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, thisinvention consists of a series of lenses or refracting elementscollectively identified generally at 10, with such large apertures thatboth eyes of the observer may comfortably view the intermediate imageprovided by the phosphor display screen 16 of an image tube 12. Thelenses are suitably mounted and maintained in a predetermined spacerelationship by a housing 14. There is no need for an adjustment ofinterpupillary distance, and only focusing need be provided for the useof this eyepiece. Focusing may be readily accomplished by providing fora variation of one or more of the air spaces between the refractiveelements of the eyepiece, or of the distance to the phosphor screen,both of which are standard practices in the art.

The eyepiece may consist of five air spaced elements or components, twoof which may be coupled together to form a cemented doublet in whichcase the eyepiece will consist of four spaced elements or components asshown in the drawing. The elements are designed to accomplish correctionof spherical and chromatic aberrations, coma and astigmatism. The fourelements in the embodiment shown are fonned from five lenses designatedrespectively I, ll, lll, IV and V progressing from the eye position atthe front of the eyepiece to the image location at the rear, with lensesll and Ill forming a doublet. The doublet is surrounded by two meniscuslenses l and W respectively on each side which are convex toward theobserver. The two eyes of an observer are located to the left of lens Ion both sides of the optical axis. A strong positive meniscus field lensV, also convex to the observer, is positioned between meniscus lens IVand the display surface 16 of image tube 12 on which the image to beviewed is displayed.

More specifically, the four air spaced elements or components of thepreferred embodiment are arranged in the following order front to rear;a positive meniscus. lens I convex to the observer; a cemented doubletconsisting of a positive lens ll and a negative lens III, the doubletbeing externally concave-convex; a positive meniscus lens lV convex tothe observer and a positive meniscus lens V convex to the observer.

All four elements are air spaced from each other by successive airspaces which are designated respectively 5,, S, and 5,, and they haveaxial thicknesses designated respectively 1,, 1,, 1,,r, and 1,.

It is characteristic of these eyepieces that lenses l and V possessstrong positive refractive power. The doublet, consisting of a positivecrown glass lens ll anda negative flint glass lens Ill and lens lV alsopossess net positive refractive power. The combination of all elementsshould be corrected for spherical and chromatic aberrations, coma andastigmatism by means readily apparent to those familiar with this art.

The equivalent focal lengths of the successive lenses l to V asconstituted in the above description are given by the mathematicalstatements contained in Table l herebelow, the focal lengths beingdesignated F, to F for lenses l to V respectively, F being the eyepiecefocal length.

4.29F F, 7.52F 2.30F F, 2.98F The values for axial thicknesses of thesuccessive lenses l to V are given in units of eyepieces focal lengthand are designated respectively r, to 1,, and the successive interlensair spaces, designated 5, to S,,, are given by the mathematicalexpressions contained in Table 2 herebelow:

The radii of the successive lens surfaces of lenses l to V which aredesignated successively R,, R R,,, R,, R [1,, R,, R and R, have thefollowing values as given in units of focal length in the mathematicalstatements in Table 3 herebelow:

One set of preferred specific values by way of specific example of thefocal lengths F, to F,, the values of the lens thicknesses r, to I, andthe values for the air spaces S, to S, and

the values of the radii R, to R, are given in Table 4 herebelow:

Although only certain embodiments of the present invention have beenshown and described in detail, other forms are possible and changes maybe made in the constructional details thereof within the specifiedlimits without departing from the spirit of the invention as defined inthe claims.

What is claimed is:

l. A biocular eyepiece comprising four air spaced components including:

a positive meniscus lens I convex to the observer;

a cemented externally concave-convex of a positive lens ll and negativelens Ill;

a positive meniscus lens lV convex to the observer,

and a positive concave-convex lens V convex to the observer. 2. Aneyepiece according to claim 1 having an aperture of about F l .0.

3. The eyepiece of claim 1 wherein the exit pupil is larger than theoverall focal length of the device.

4. The eyepiece of claim 1 including means for establishing opticalcontact between said element V and an intermediate imaging means.

5. The eyepiece of claim 4 wherein said means comprises a piano-concaveoptic faceplate.

6. An eyepiece with substantially large apertures to permit biocularviewing of fields of view up to 60 from an eye relief distance of up toabout mm comprising five air spaced elements including:

a positive meniscus lens convex to the observer designated a cementedexternally concave-convex doublet consisting of a positive and anegative lens designated ll and [II respectively,

a positive meniscus lens convex to the observer IV, and

a positive concave-convex lens designated V;

the value for the constructional data for said eyepiece being givensubstantially in the tables of mathematical expressions herebelowwherein F, to F 5 respectively represent the equivalent focal length oflenses l to V, R, to R, respectively represent the radii of thesuccessive lens surfaces of the lens I to V, I, to I, represent theaxial thickness of lenses 1 to V, and S, to S, represent the successiveinterlens air spaces between designated thelenses:

F, 2.667 t, 0.157 s, 0.035 F, I L323 t: I 0.313 S, 0.256 F,, I l.4l8 t,0.087 S: 0.017

F, I 5.450 t, I 0.157 F, I 2.597 t, 0.l22

doublet consisting I

1. A biocular eyepiece comprising four air spaced components including:a positive meniscus lens I convex to the observer; a cemented externallyconcave-convex doublet consisting of a positive lens II and negativelens III; a positive meniscus lens IV convex to the observer, and apositive concave-convex lens V convex to the observer.
 2. An eyepieceaccording to claim 1 having an aperture of about F/1.0.
 3. The eyepieceof claim 1 wherein the exit pupil is larger than the overall focallength of the device.
 4. The eyepiece of claim 1 including means forestablishing optical contact between said element V and an intermediateimaging means.
 5. The eyepiece of claim 4 wherein said means comprises aplano-concave optic faceplate.
 6. An eyepiece with substantially largeapertures to permit biocular viewing of fields of view up to 60* from aneye relief distance of up to about 80 mm comprising five air spacedelements including: a positive meniscus lens convex to the observerdesignated I, a cemented externally concave-convex doublet consisting ofa positive and a negative lens designated II and III respectively, apositive meniscus lens convex to the observer designated IV, and apositive concave-convex lens designated V; the value for theconstructional data for said eyepiece being given substantially in theTables of mathematical expressions herebelow wherein F1 to F5respectively represent the equivalent focal length of lenses I to V, R1to R9 respectively represent the radii of the successive lens surfacesof the lens I to V, t1 to t5 represent the axial thickness of lenses Ito V, and S1 to S3 represent the successive interlens air spaces betweenthe lenses: F1 2.667 t1 0.157 S1 0.035 F2 1.323 t2 0.313 S2 0.256 F31.418 t3 0.087 S3 0.017 F4 5.450 t4 0.157 F5 2.597 t5 0.122 R1 0.9857 R22.293 R3 0.8067 R4 38.52 R5 0.9523 R6 0.4810 R7 0.4908 R8 0.4305 R90.5009